1. Field of the Invention
The present invention relates to a device for purifying the exhaust gas of an internal combustion engine.
2. Description of the Related Art
Exhaust gas discharged from a Diesel engine contains a relatively large quantity of exhaust particulates, the principal component of which is carbon. Therefore, environmental pollution is caused by these exhaust particulates contained in the exhaust gas. Accordingly, it is preferable to remove these exhaust particulates before the exhaust gas is discharged from the engine into the atmosphere. In order to accomplish the above object, there is provided a filter to collect the particulates in the exhaust passage of a Diesel engine.
While the Diesel engine is operated, the quantity of particulates collected by the filter increases. In accordance with the increase in the quantity of particulates collected by the filter, the exhaust resistance is increased, so that the engine performance is deteriorated. In order to prevent the deterioration of the engine performance, particulates collected by the filter must be periodically burned so as to regenerate the filter.
In the process of combustion conducted in a Diesel engine, not only fuel but also engine oil, which has intruded into cylinders, is burned. Accordingly, oxides and sulfides of calcium and phosphorus, which are components of engine oil, are produced. Usually, particulates contain the thus produced oxides and sulfides. It is very difficult to burn the oxide and sulfide of calcium and phosphorus. Therefore, in the regeneration of the filter, the oxide and sulfide of calcium and phosphorus remain and accumulate on the filter in the form of ashes, so that the exhaust resistance of the filter is increased.
Japanese Unexamined Patent Publication No. 4-31613 discloses an exhaust gas processing device in which ashes are removed from the filter as follows. There is provided a bypass passage which bypasses the filter, and this bypass passage is connected with an exhaust passage on the upstream side of a branch point from which an exhaust gas recirculating passage branches. In this bypass passage, there is provided a first closing valve which is normally closed. Between the branch point of the exhaust gas recirculating passage and the branch point of the bypass passage in the exhaust passage, there is provided a second closing valve which is normally opened.
In this exhaust gas processing device, exhaust gas usually passes through the exhaust passage, and particulates are collected by the filter. When the temperature of exhaust gas is raised, the collected particulates are burned, so that the filter can be regenerated. In accordance with the progress of regeneration of the filter, once a quantity of ashes remaining in the filter is increased, the first closing valve is opened, and at the same time the second closing valve is closed. Due to the foregoing, exhaust gas flows into the bypass passage and reaches the downstream side of the filter in the exhaust passage. At this time, a negative pressure of the intake system acts in the exhaust passage on the upstream side of the filter via the exhaust gas recirculating passage. Accordingly, a back flow of exhaust gas is produced in the filter by the action of negative pressure. This exhaust gas processing device is intended to remove ashes from the filter by this back flow of exhaust gas. By the way, exhaust gas discharged from a spark ignition engine also contains the particulates so that such a filter can be arranged in the exhaust passage thereof.
It is possible to remove ashes from the filter by the above-mentioned exhaust gas processing device. However, the exhaust gas processing device requires two closing valves and a driving unit to drive them. Accordingly, the manufacturing cost is raised, and further the structure of the exhaust system becomes complicated.